Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
  • G01V5/20—Detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects
  • G01V5/22—Active interrogation, i.e. by irradiating objects or goods using external radiation sources, e.g. using gamma rays or cosmic rays

Definitions

• the present disclosure is generally related to a gamma ray detector for security
• a device in a particular embodiment, includes means for providing a gamma ray detector for security applications with large acceptance, good energy resolution, directional sensitivity, and moderate cost, with the large acceptance being at least a thousand times greater than a conventional acceptance of a conventional gamma ray detector.
• the device also includes means for operating the gamma ray detector for security applications for at least one of detection of smuggled nuclear weapons and nuclear materials, being carried in an airplane for aerial searches for radioactive materials, detecting a signature of passage of nuclear powered vessels and ships carrying nuclear cargos, examining unidentified radioactive waste and cargo, and tracking cosmic ray muons allowing entirely passive scanning of any container, no matter how well the container is shielded, using a large size and good position resolution of the gamma ray detector for security applications.
• a method in another particular embodiment, includes steps for providing a gamma ray detector for security applications with large acceptance, good energy resolution, directional sensitivity, and moderate cost, with the large acceptance being at least a thousand times greater than a conventional acceptance of a conventional gamma ray detector.
• the method also includes steps for operating the gamma ray detector for security applications for at least one of detection of smuggled nuclear weapons and nuclear materials, being carried in an airplane for aerial searches for radioactive materials, detecting a signature of passage of nuclear powered vessels and ships carrying nuclear cargos, examining unidentified radioactive waste and cargo, and tracking cosmic ray muons allowing entirely passive scanning of any container, no matter how well the container is shielded, using a large size and good position resolution of the gamma ray detector for security applications.
• FIG. 1 is a diagram illustrating an embodiment of an apparatus including means for providing a gamma ray detector for security applications with large acceptance, good energy resolution, directional sensitivity, and moderate cost, with the large acceptance being at least a thousand times greater than a conventional acceptance of a conventional gamma ray detector and means for operating the gamma ray detector for security applications for at least one of detection of smuggled nuclear weapons and nuclear materials, being carried in an airplane for aerial searches for radioactive materials, detecting a signature of passage of nuclear powered vessels and ships carrying nuclear cargos, examining unidentified radioactive waste and cargo, and tracking cosmic ray muons allowing entirely passive scanning of any container, no matter how well the container is shielded, using a large size and good position resolution of the gamma ray detector for security applications; and
• FIG. 2 is a flow diagram of an illustrative embodiment of a method including steps for providing a gamma ray detector for security applications with large acceptance, good energy resolution, directional sensitivity, and moderate cost, with the large acceptance being at least a thousand times greater than a conventional acceptance of a conventional gamma ray detector and steps for operating the gamma ray detector for security applications for at least one of detection of smuggled nuclear weapons and nuclear materials, being carried in an airplane for aerial searches for radioactive materials, detecting a signature of passage of nuclear powered vessels and ships carrying nuclear cargos, examining unidentified radioactive waste and cargo, and tracking cosmic ray muons allowing entirely passive scanning of any container, no matter how well the container is shielded, using a large size and good position resolution of the gamma ray detector for security applications.
• the apparatus 100 includes means for providing a gamma ray detector for security applications with large acceptance, good energy resolution, directional sensitivity, and moderate cost, with the large acceptance being at least a thousand times greater than a conventional acceptance of a conventional gamma ray detector 110 and means for operating the gamma ray detector for security applications for at least one of detection of smuggled nuclear weapons and nuclear materials, being carried in an airplane for aerial searches for radioactive materials, detecting a signature of passage of nuclear powered vessels and ships carrying nuclear cargos, examining unidentified radioactive waste and cargo, and tracking cosmic ray muons allowing entirely passive scanning of any container, no matter how well the container is shielded, using a large size and good position resolution of the gamma ray detector for security applications 120.
• the method 200 includes steps for providing a gamma ray detector for security applications with large acceptance, good energy resolution, directional sensitivity, and moderate cost, with the large acceptance being at least a thousand times greater than a conventional acceptance of a conventional gamma ray detector 210 and steps for operating the gamma ray detector for security applications for at least one of detection of smuggled nuclear weapons and nuclear materials, being carried in an airplane for aerial searches for radioactive materials, detecting a signature of passage of nuclear powered vessels and ships carrying nuclear cargos, examining unidentified radioactive waste and cargo, and tracking cosmic ray muons allowing entirely passive scanning of any container, no matter how well the container is shielded, using a large size and good position resolution of the gamma ray detector for security applications 220.
• the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as those that are inherent therein. While the present invention has been depicted, described and is defined by reference to exemplary embodiments of the present invention, such a reference does not imply a limitation of the present invention, and no such limitation is to be inferred. The present invention is capable of considerable modification, alteration, and equivalency in form and function as will occur to those of ordinary skill in the pertinent arts having the benefit of this disclosure. The depicted and described embodiments of the present invention are exemplary only and are not exhaustive of the scope of the present invention.
• PROJECT TITLE Very Large Acceptance Gamma Ray Detector
• Gamma ray detectors for security applications should have large acceptance, good energy resolution, directional sensitivity and moderate cost.
• the large acceptance detectors now in use have extremely poor energy resolution and almost no directional sensitivity.
• the technique we propose to determine both the gamma energy and direction is to use many layers of low-cost segmented plastic scintillator and/or scintillating aerogel alternating with planes of photodetectors. Multiple wells in the body allow the photons to initially interact deep in the volume to greatly improve the collection of the backward scattered photons. By using a very large volume and relatively low density, the detector allows the Compton scattered photons to travel far enough to provide good direction information. From their energies and the locations of the electrons, one can infer the direction and energy of the incident gamma ray. Its acceptance is thousands of times greater than that of a conventional detector.
• the detector can be used to detect smuggled nuclear weapons and nuclear materials at ports-of-entry and major cargo terminals. It could be carried in an airplane for aerial searches for radioactive materials. It might even prove useful to detect a signature of the passage of nuclear powered vessels and ships carrying nuclear cargos. It may also be used as an aid in examining unidentified radioactive waste and cargo. Its large size and good position resolution makes it an excellent tracker of cosmic ray muons, allowing entirely passive scanning of any container, no matter how well it is shielded.
• KEY WORDS gamma rays, tracking, nuclear detection, nuclear proliferation
• a new type of gamma ray detector offers a promise of roughly a 1000-fold increase in sensitivity and directional capability for finding hidden nuclear materials.
• This proposal is to study a novel gamma ray detector concept based on a large, relatively low density volume containing a high density of next generation optical detectors, providing both directional information, energy resolution, and a very large acceptance.
• HPGe hyper-pure germanium detectors
• Nal sodium iodide
• G4beamline[l] a widely used tool to simulate the interaction of particles and photons with matter and based on Geant4[2] was used to simulate a beam of 100 1 MeV photons interacting with the largest readily available conventional gamma ray detectors, a 10x10" Nal ( Figure 1) and a 7x7cm HPGe ( Figure 2); photons are shown in green and electrons in red.
• Both the Nal and HPGe detectors sacrifice essentially all directional information and are relatively slow devices; in both cases, some of the energy escapes.
• both the direction and energy of the incoming photons may be determined. This requires a finely granulated detector with excellent timing; only the recent advent of extremely fast, large area, inexpensive photodetectors makes such a device possible. While the energy resolution will never approach that of HPGe detectors, the large volume of the detector will collect thousands of times more photons.
• Figure 1 G4beamline simulation of 100 lMeV gamma rays entering a 10x10 inch Nal cylindrical scintillator on axis from the right.
• Figure 2 G4beamline simulation of 100 lMeV gamma rays entering a 7 x 7 cm HPGe detector on axis from the right.
• Common plastic scintillator may be approximated by C with a density of 1 g/cm ; at the energies of interest the cross section is dominated by Compton scattering. This scintillator is suitable for better than 1 ns timing. Due to multiple scattering, it is unlikely that both the first scattered electron's direction and energy could be measured well enough to be useful, but if the locations, order, and energies of all the scattered electrons could be measured sufficiently, one ought to be able to reconstruct the energy and direction of the initial photon.
• One of the design criteria is the thickness of the layers in the detector, to be determined by simulations in Phase I.
• Gamma rays are able to penetrate many layers of plastic, but the Compton electrons' ranges are energy-dependent and are in the mm range for the relevant energies.
• a plot of electron range in polystyrene, shown in Figure 3b, helps set the scale for the thickness of the layers.
• An electron with energy of 1 MeV has a range of about 5mm of plastic; while the range for the highest possible energy Compton electron from a 2 MeV photon is about 10mm.
• VLAGRD Very Large Acceptance Gamma-Ray Detector
• Figure 5 illustrates 100 lMeV gamma rays entering a 4m x2.54m VLAGRD through a 1" bore hole.
• the left hand figure in Figure 5 shows how the gamma rays scatter as they move through the detector.
• the gamma rays lose energy as they undergo interactions.
• the gammas penetrate further into the forward hemisphere, but a significant number scatter backwards. All of the gammas are contained in the detector.
• the right hand part of Figure 5 shows a plot of the locations of the points at which Compton scattered electrons are produced by the gammas.
• This plot shows that the Compton-scattered electrons are confined to a region of approximately -700 mm ⁇ x ⁇ +600 mm and 500 mm ⁇ z ⁇ 2300 mm. This also shows the justification for employing a detector as large as 2.5 m deep and several m in transverse size.
• Figure 6 compares the Compton electrons produced in a VLAGRD with a no bore and the one with a 1 inch bore, in which 10,000 1 MeV photons impinge on the axis of the bore hole.
• no hole about 30% of the events lose some fraction of their energy outside the VLAGRD, mostly in the backward direction.
• the bore hole only about 0.3% of the gammas lose energy outside of the detector.
• a realistic design which considers the target distance, depth and width of field, and rate should lie somewhere in between these extremes.
• Figure 5 G4beamline simulation of 100 lMeV gamma rays entering a 4 m diameter x 2.54 m thick VLAGRD on axis through a 1" hole from the right. The figure on the left shows the
• An alternative to boring holes may be to cast the scintillator sheets on a mold that has protrusions corresponding to the holes in the sheets. trajectories of gammas as they interact in the detector.
• the right hand figure shows a plot of the of x-position (horizontal) vs z-position (axial) of the points at which Compton scattered electrons originate in detector The blue lines indicate the outer boundary of the detector.
• Figure 6 Simulation of 1000 1 MeV gamma rays entering from the right into a 4 x 2.54 m VLAGRD with and without a central 1" bore.
• VLAGRD Surrounding the VLAGRD is an idealized invisible closed cylinder to tabulate escaped photons.
• the Comp ton-scattered electrons are shown as green dots; the escaped forward photons (-0.3%) in mauve, and no photons escape backward.
• the Compton electrons are shown in red, no photons escape forward, and the backward escaped photons (-30%) are shown as turquoise points.
• Figure 7 shows scatter plots of radial distance (r) vs time difference (t - to) for Compton- scattered electrons and kinetic energy of Compton- scattered electrons vs time difference.
• the time difference is the difference between the time a Compton scattering occurs (t) and the time of the first interaction that produces a Compton electron (t 0 ).
• the r vs (t - 1 0 ) plot shows that most of the secondary Compton scatters occur within 5 ns of the primary interaction, with some correlation.
• the kinetic energy vs (t - to) plot shows how the energy degrades as the gamma ray propagates.
• the strong peaking at times « Ins are due mainly to the high energy Compton scatters. There is a large spread in times for the very low energy photons (KE « 0.1 MeV). For (t - 1 0 ), there are no photons seen with energies > -0.02 MeV.
• Figure 7 Compton electrons from a simulation of 100 1 MeV gamma rays entering a 4m diameter x 2.54 m thick VLAGRD in a 1" central bore; left shows distance to axis vs. time from 1 st interaction, while right shows kinetic energy vs. time from 1 st interaction.
• Figure 8 shows a section of a detector consisting of an array of plastic scintillator (or doped aerogel scintillator) plates arranged in planes that are stacked to form modules. Each plate has a silicon photomultiplier (SiPM) mounted directly and optically coupled to the scintillator plate.
• SiPM silicon photomultiplier
• the entire detector is made of scintillator, except for the SiPM photon sensors.
• SiPMs are available from a number of industrial suppliers. SiPMs operate a low voltage ( ⁇ 50VDC) and a relatively low cost. SiPMs have good time resolution ( ⁇ few ns), well matched to the signals from plastic scintillators.
• the planes closest to the source can have holes bored through the scintillator planes to allow some of the gammas to penetrate into the center of the array of planes in order to capture the backward-going products of the interactions of the gammas in the detector.
• the space resolution improves as the inverse size of the scintillator plates, and the cost increases with the number of SiPMs, which, in turn, depends on the number of layers ( Figure 8).
• Figure 8 A conceptual version of a section of a VLAGRD, based on a large array of scintillators with a silicon photomultiplier (SiPM) attached to each piece of scintillator. Holes in the first set of scintillator planes allow gammas to penetrate deeper.
• SiPM silicon photomultiplier
• a further improvement in the detector is to utilize photo-detectors that are being developed as a new generation of fast, high-resolution large-area, low-cost micro-channel plates (MCPs), as shown in Figures 9 and 10.
• MCPs micro-channel plates
• This type of detector is being developed by a collaboration of groups from universities, National Labs, and businesses [5]. These detectors are being designed for time resolutions that are ⁇ 10ps and position resolutions -0.5 mm, both of which are better than we anticipate for our requirements, but relaxing both the time and space resolution requirements would reduce the cost of the associated electronics.
• Phase I we propose to do a feasibility study of using this type of detector. We consider that this concept may be more suitable for use in the future when this type of detector becomes available for use in this application.
• FIG. 9 A representation of a VLAGR using the MCP-like modules is shown in Figure 9. Planes of scintillator are alternated with planes of MCP photodetectors; light produced in the scintillators is converted to electrical signals by the MCPs. The scintillator planes need not be segmented into sub-units as in the first arrangement, but holes may be bored in the first layers of the scintillators as in the first arrangement. No holes need be cut into the MCP detectors themselves.
• Figure 10 shows a schematic representation of the functioning of the MCP element in its fast time resolution mode. In this example, but not in a VLAGRD, the entrance window is used as a Cherenkov radiator.
• the radiated photon hits a photocathode and produces a photoelectron that is multiplied in the MCPs, and the signal is collected at the anode plane, which is segmented to provide position resolution.
• the light is produced in the scintillator, and the window is clear glass, necessary for maintaining a vacuum required for the MCP elements.
• Figure 10 Schematic view of a MCP detector. The left-hand figure illustrates the principle of operation. Light produced in the radiator impinges on a photocathode. The
• Phase II a portion (with a volume of at least several cubic feet) of a large gamma detector will be constructed, tested, and compared to simulations.
• This proposal addresses an opportunity to develop a very large, relatively low cost detector. It would have an enormous acceptance for gamma rays, and could quickly detect and locate radioactive sources.
• the same detector may also serve as a relativistic particle tracker, for example, as a cosmic ray tracker to passively scan cargo containers, and could do both tasks simultaneously. With small modifications, it could also be used as a neutron detector, and would be very good at neutron/gamma separation. It could also be used as the detector with active interrogation systems, greatly reducing the require dose delivered to the cargo.
• Phase I work plan The main purpose of the Phase I work plan is to develop a conceptual design for very large acceptance gamma ray detector with achievable large area photodetectors.
• Muons, Inc. is currently a member of the LAPPD collaboration and is exploring other applications for these new photo-detectors, including using them for non-magnetic particle spectrometer systems and for cosmic ray muon scanning of containers.
• Muons, Inc Scientist and Principal Investigator: Dr. Kevin Beard earned his Ph.D. at Michigan State University's National Superconducting Cyclotron Laboratory and has participated in many nuclear experiments at a number of facilities. He has experience designing and using a variety of nuclear detectors and performed many of the early simulations for the Gammasphere detector. Prior to joining Muons, Inc., he worked the Jefferson Lab's Free Electron Laser, and has been a member of the muon collider collaboration for some years. He is an active member of the LIPSS dark matter search collaboration and wrote all the software to analyze the output of the CCD cameras used in that experiment. His office is at Jefferson Lab in Newport News, VA.
• Muons, Inc. currently shares facilities with MuPlus, Inc. This includes our corporate headquarters, a building of approximately 4000 square feet of floor space in Batavia, IL, a short drive from Fermilab, which is used as office space conference rooms, workshop area, and living quarters as needed. We also share office space with MuPlus, Inc. in Wilson Hall at Fermilab (Batavia, IL) and in the ARC building at Jefferson Lab (Newport News, VA). We have several high-performance personal computers and workstations with high-speed net access and sufficient computer power to perform simulations and CAD work.
• G4beamline A "Swiss Army Knife" for Geant4, optimized for simulating beamlines, http://g4beamline.muonsinc.com

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